1. Field of the Invention
The invention relates to a semiproduct for the manufacture of light conducting fibers, especially those for the multimode conduction of light signals, to a method of making the semiproduct, and to a light conducting fiber made from the semiproduct.
2. Discussion of the Prior Art
In the manufacture of light conducting fibers from glassy material, it is known to use as semiproducts tubes of small wall thickness and thin rods (German "Offenlegungsschrift" No. 2,418,168). From these two semiproducts, a light conducting fiber is made by inserting a thin rod into a tube whose inside diameter is slightly larger than the diameter of the rod. Both are heated at one end by means of a heat source to such a degree that a light conducting fiber of a diameter of approximately 50 .mu.m can be drawn from the heated zone. The tubular material forms the jacket of the light conducting fiber, whose core consists of the rod material. These light conducting fibers, manufactured by the "rod-and-tube process", thus require two semiproducts.
In the use of these semiproducts for the manufacture of light conducting fibers, considerable difficulty consists in cleansing the inside surface of the tubes (the inside diameter is of the order of a few millimeters) such that no residual impurities will remain which result in the formation of small bubbles at the interface between the jacket and core when the fibers are drawn, which cause increased scatter losses.
From U.S. Pat. No. 3,711,262 it is known to manufacture light conducting fibers having a core and jacket by coating the mechanically and flame polished inside wall of a glass tube with a thin film of a vitreous material which forms the core of the light conducting fiber. The internally coated glass tube is then heated to the drawing temperature and drawn to form the light conducting fiber. Quartz glass, for example, is used as the material of the tube, the internal coating in that case consisting of a silica glass which is doped with an oxide which increases the index of refraction.
The use of synthetic quartz glass containing less than 5 ppm of hydroxyl ions, less than 5 ppm of metal ions and less than 12 scattering centers visible to the naked eye per cubic centimeter, as a material for the core or jacket of a light conducting fiber is known from German "Auslegeschrift" No. 2,202,787.
Lastly, from U.S. Pat. No. 3,869,194 it is known that light conducting fibers with a core of high-purity quartz glass and a jacket of fluoride glass or fluoride-containing quartz glass have proven unsuitable for optical signal transmission.